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Exact solutions show how concept directions align during training, not just after it.

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T0 review · grok-4.5

2026-07-13 06:18 UTC pith:YFIZE6ZB

load-bearing objection Exact abstraction trajectories under 2FS linear nets are the real contribution; the GELU-ablation application is a useful but under-proved leap from the infinite-width attenuation law. the 2 major comments →

arxiv 2607.08843 v1 pith:YFIZE6ZB submitted 2026-07-09 cs.LG

How are linear representations learned? Exact solutions to the dynamics of abstraction

classification cs.LG
keywords linear representation hypothesisabstraction dynamicsdeep linear networkskernel Riccati dynamicsattenuation lawlinear probesconcept geometryinfinite-width NNGP
verification ladder T0 review T1 audit T2 compute T3 formal T4 reserved

The pith

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

This paper asks how linear concept directions form while a network trains, not merely whether they exist at the end. In a minimal linear network with two binary factors, the authors solve the full trajectory of abstraction—the cosine alignment of concept vectors across contexts. Terminal abstraction is fixed by the geometric mean of input and target noise ratios; depth improves it when targets are cleaner than inputs; small initialization lets abstraction overshoot and stay near-perfect for a long time. Nonlinearities (erf, ReLU) change the dynamics and, at any fixed time, weaken abstraction in post-activation features relative to preactivations. Evidence in vision and language models, plus a simple local ablation that improves probe transfer, shows the laws still matter outside the solvable setting.

Core claim

Under a minimal linear network with two-factor symmetry and optimal readout, abstraction has an exact closed-form trajectory. Its terminal value is set only by the geometric mean of input and target inverse signal-to-noise ratios; depth and initialization separately control layerwise growth and peak overshoot. In infinite-width nonlinear nets the same geometry is reshaped by the nonlinearity, and both erf and leaky ReLU attenuate abstraction so that feature abstraction never exceeds preactivation abstraction.

What carries the argument

Abstraction score α: the cosine similarity of a concept direction measured in two contexts, rewritten as a function of the signal-to-noise ratio of two eigenmodes (S versus SC) of a five-entry two-factor-symmetric kernel. Exact scalar ODEs for those modes yield the terminal law, depth interpolation, and attenuation factor.

Load-bearing premise

The whole exact theory needs the data, targets, and initial features to obey a strong two-factor symmetry so that every kernel collapses to five numbers and the same five modes; without that symmetry the closed-form trajectory disappears.

What would settle it

Train a deep ReLU or transformer stack on data that clearly violates two-factor symmetry and check whether terminal abstraction still tracks the geometric-mean formula, whether deeper layers are more abstract, and whether ablating the local nonlinearity still raises feature abstraction and probe transfer.

Watch this falsifier — get emailed when new claim-graph text bears on it.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit.

Referee Report

2 major / 6 minor

Summary. The paper develops a dynamical theory of “abstraction” (cosine alignment of context-specific concept vectors) under the linear representation hypothesis. In a two-layer linear network with variable-projected readout and two-factor symmetry (2FS) kernels, the authors reduce gradient flow to scalar mode ODEs and obtain an exact implicit trajectory for abstraction (Theorem 3). From this they derive a terminal law α∞ determined by the geometric mean of input and target inverse-SNRs (Theorem 4), overshoot and initialization-scale control of peak abstraction (Theorem 5, Proposition 6), and depth laws for layerwise interpolation and terminal abstraction under balanced deep linear networks (Theorem 7). They extend to infinite-width two-layer erf and leaky-ReLU networks, prove an attenuation law |αK| ≤ |αQ| for features vs preactivations (Theorem 8 / C.2), and characterize how ReLU terminal abstraction depends more on input than target geometry. Empirical checks include ResNets on 3dshapes, local GELU ablation in DINOv3 and Gemma 4, and macaque V4 vs IT.

Significance. If the linear results hold under the stated axioms, they give a rare closed-form account of how linear concept geometry evolves during training—not only at convergence—linking data/target geometry, depth, and rich/lazy initialization to abstraction trajectories. That is a clear advance over asymptotic perfect-abstraction results. The attenuation law and ReLU vs erf phase diagrams offer a mechanistic explanation for known nonlinearity effects on abstract geometry. Strengths include exact solutions (Riccati reduction, eigenvalue ODEs, Theorems 3–7), careful infinite-width nonlinear maps and proofs (Appendix C), and falsifiable qualitative predictions tested on ResNets, open transformers, and neural data. The applied GELU-ablation result is modest but practically relevant for probing/steering if the effect is robust.

major comments (2)
  1. §5 and abstract claim evidence for the attenuation law and improved probe generalization via local GELU ablation (Fig. 5). Theorem 8 / Theorem C.2 only prove |αK| ≤ |αQ| for infinite-width Gaussian preactivations with exact 2FS kernels under erf or leaky ReLU (Eq. 15; §C.5). Residual streams in DINOv3/Gemma are finite-width, multi-layer, non-Gaussian, and not 2FS; GELU is not among the verified nonlinearities. The manuscript should explicitly separate the theorem’s hypotheses from the empirical intervention, avoid language that presents Fig. 5 as a consequence of Theorem 8, and add controls (e.g., ablating other MLP components, random linear maps, or non-concept baselines) so the ~1pp probe gain is not over-attributed to the attenuation mechanism.
  2. Assumption 2 (2FS; §2.2, §A.3) is load-bearing for every exact scalar ODE and closed-form law (Proposition 2, Theorems 3–7). It is imposed via group invariance rather than derived for realistic data. The ResNet/3dshapes checks (§3.3, Fig. 3) are qualitative and still use a 2×2 factor design. The paper should state more sharply which predictions are expected to survive without 2FS (e.g., qualitative depth/init trends) versus which are 2FS-specific (exact α∞ formula, arctanh layerwise interpolation), and ideally report a controlled symmetry-breaking experiment (e.g., unbalanced class sizes or non-orthogonal Fourier components) to bound sensitivity.
minor comments (6)
  1. Assump. 1 (variable-projected readout) is better motivated than a frozen readout, and §A.4/Fig. 7 help, but early-training discrepancies should be flagged when interpreting non-monotonic α(t) near initialization.
  2. Setting 2 (layerwise balancing) for depth results (§3.2, §B.8) is standard but strong; a short remark on how unbalanced init or SGD noise would perturb Eq. (13)–(14) would help readers.
  3. Proposition 1’s Gaussian score approximation is used to link α to probe transfer; state when the approximation fails (heavy-tailed residual streams, multi-token concepts).
  4. Fig. 1E / 2FS entry notation (ad, a2, a1s, a1c, a0) is dense; a small table mapping entries to modes S/SC would improve readability.
  5. Related work on Word2Vec analogies and LRH is good; a one-sentence contrast with Jiang et al. [4] on dynamics vs asymptotics in the main text (not only appendix) would help orientation.
  6. Gemma results: report confidence intervals or paired tests for the 17/18 probe improvements; French–Spanish decrease should be discussed briefly.

Circularity Check

0 steps flagged

No significant circularity: terminal, depth, and attenuation laws are derived from gradient-flow ODEs under stated assumptions, not forced by definition or fitted targets.

full rationale

The paper's load-bearing analytic claims (Theorems 3–5, 7–8; Propositions 2, 6, 10–11) are obtained by reducing gradient flow under Assumps. 1–2 (and Settings 1–3) to scalar mode ODEs, then solving or analyzing those ODEs. Terminal α∞ follows from late-time √t growth of λS and λSC (Theorem B.1 / Theorem 4); depth laws from balanced gain factorization in arctanh-SNR space (Theorem 7); attenuation from NNGP maps with nonnegative power-series coefficients (Theorem C.2). None of these equal their inputs by construction: α is defined as cosine similarity of concept vectors, then shown to obey dynamics whose fixed points and bounds are nontrivial functions of data geometry, depth, init scale, and nonlinearity. Variable projection and 2FS are explicit modeling assumptions that enable closed form, not renamings of the claimed trajectory. Empirical applications (GELU ablation, macaque hierarchy) are independent tests, not fitted-then-predicted quantities. Self-citations to linear-network dynamics literature supply techniques, not uniqueness theorems that force the abstraction laws. Score 0 is therefore appropriate.

Axiom & Free-Parameter Ledger

3 free parameters · 6 axioms · 3 invented entities

Central analytic claims rest on standard gradient-flow math plus two strong domain modeling assumptions (variable-projected readout; 2FS kernels), layerwise balancing for depth, and infinite-width Gaussian NNGP structure for nonlinearities. No free parameters are fitted to force the terminal law; empirical sections use ordinary training hyperparameters. Invented entities are definitional constructs (abstraction score, 2FS modes, effective target kernel), not new physical objects.

free parameters (3)
  • ridge γ / readout regularization
    Appears in the reduced loss and spectral filter for M(Q); shapes dynamics but terminal α∞ is independent of γ in the linear interior case. Chosen by experimenters in simulations.
  • initialization scale κ (or σ_w²)
    Controls α_max and rich/lazy behavior (Theorem 5); swept empirically rather than fitted to a claimed universal constant.
  • nonlinearity parameters β (erf), ω (leaky ReLU)
    Enter kernel maps and ReLU critical noise η_crit(ω); set by model choice, not fit to recover α∞.
axioms (6)
  • standard math Gradient flow on MSE with ridge on readout only; features Z = WX (linear) or infinite-width Gaussian preactivations.
    Standard continuous-time training model used throughout §2–§4.
  • domain assumption Assumption 1: readout always at ridge optimum Wr*(Z) (variable projection / infinite readout learning-rate limit).
    Enables reduced loss L*(Z) and Riccati dynamics; compared to finite-rate GD in §A.4 but not exact for early training.
  • domain assumption Assumption 2 (2FS): Σx, Σy, Q(0) invariant under (Sn)^4 ⋊ (Z2)^2, yielding five-entry kernels and modes I,S,C,SC,G.
    Load-bearing for simultaneous diagonalization and scalar ODEs; motivated by orthogonal Walsh components of two binary latents.
  • domain assumption Setting 2: layerwise balanced gains per mode so λ_m^(ℓ) = λ_m^(0) u_m^{2ℓ}.
    Required for depth ODE and arctanh interpolation (Theorem 7); standard balanced deep-linear assumption.
  • domain assumption Infinite-width NNGP: feature kernel is E[ϕ(zi)ϕ(zj)] for z ~ N(0,Q); 2FS preserved under erf/L-ReLU.
    Foundation of §4 kernel maps, dynamics, and attenuation proof.
  • ad hoc to paper Setting 1 / Setting 3 regimes for overshoot and ReLU terminal analysis (signal-dominant; centered signal-balanced).
    Restrict phase diagrams and overshoot theorems to analytically tractable slices of geometry.
invented entities (3)
  • Abstraction score α (cosine of context-specific concept vectors) independent evidence
    purpose: Continuous dynamical variable measuring concept-direction alignment; links to LRH cone-linearity and probe transfer.
    Equivalent to neuroscience parallelism; continuous relaxation of cone-linearity (§A.2). Independent evidence via probe generalization and brain analyses.
  • 2FS eigenmodes and inverse-SNR ν = λ_SC/λ_S no independent evidence
    purpose: Reduce kernel dynamics to scalar signal/noise ODEs and closed-form α laws.
    Defined from symmetry assumption; useful bookkeeping rather than a new physical object.
  • Effective target kernel M(Q) and nonlinear gain R_ϕ no independent evidence
    purpose: Encode how targets (and nonlinearities) drive feature learning after readout projection.
    Derived objects from the reduced loss and Stein/NNGP calculations.

pith-pipeline@v1.1.0-grok45 · 65527 in / 3640 out tokens · 34954 ms · 2026-07-13T06:18:33.353821+00:00 · methodology

0 comments
read the original abstract

In artificial and biological neural networks, concepts are often encoded as consistent linear directions in representation space. In deep learning, this idea is known as the linear representation hypothesis and underpins many interpretability and control methods based on linear probes, from concept detection to activation steering. Yet while prior work has studied whether such directions should exist $\textit{after}$ training, the dynamics of how they emerge $\textit{during}$ training remain poorly understood. Here, we develop a framework to study the alignment of concept directions during training - a process we call "abstraction". In a minimal linear network setting, we obtain exact solutions for the full trajectory of abstraction. These solutions reveal key analytic principles governing abstraction: (i) data and target geometry jointly determine abstraction at the end-of-learning, (ii) abstraction improves with network depth, and (iii) initialization scale controls the maximum abstraction reached during training. Extending our theory to nonlinear networks, we analyze how the choice of nonlinearity affects abstraction dynamics: erf networks approximate the linear theory, while abstraction in ReLU networks depends less on target geometry and more on input geometry. Across both, we prove a striking attenuation law: both nonlinearities weaken abstraction in activations relative to preactivations. We find evidence for this law in open models (DINOv3, Gemma 4) and apply our theory to improve linear probe generalization in LLMs. Together, our results provide a dynamical theory of abstraction with implications for interpretability and control.

Figures

Figures reproduced from arXiv: 2607.08843 by Andrew M. Saxe, Peter E. Latham, William W. Yang.

Figure 1
Figure 1. Figure 1: Problem setting (see §2 for more details). We consider a minimal solvable setting with two binary concepts (shape, color) giving us four classes C := { , , , }. (A) Schematic of perfectly abstract representation where shape concept vectors ured and ublue have cosine similarity αshape = 1. (B) In the minimal model, introducing an orthogonal interaction mode (SC, green) with eigenvalue λSC lowers the abstrac… view at source ↗
Figure 2
Figure 2. Figure 2: Analytic laws governing abstraction dynamics in the minimal model. All solid colored lines correspond to numerical simulations. (A) shows trajectory of α(t) in an L = 1 hidden layer network for varying weight initialization. For all initializations, α(t) converges to the red terminal value Theorem 4. Meanwhile, the maximum abstraction reached over the trajectory decreases with initialization scale (Theorem… view at source ↗
Figure 3
Figure 3. Figure 3: Empirical validation of the minimal model in convolutional networks. (A) We train small ResNets on a task constructed from the 3dshapes dataset [21] using binary shape and color latents, with n = 1000 examples per class while nuisance variables (floor color, wall color, size, orientation) vary randomly. For panels B-D, error bars show SEM across 20 seeds. (B) Maximum abstraction in last layer decreases wit… view at source ↗
Figure 4
Figure 4. Figure 4: Effect of nonlinearities on terminal abstraction. Panels A-D show phase diagrams of αQ,∞ as a function of noise strength η• := ν(Σ•)/(1 + ν(Σ•)) for inputs and targets. (A) Erf terminal abstraction with weak regularization (β = 1, γ = 1000) appears qualitatively well-described by the linear theory. (B) Linear theory from Theorem 4. (C-D) L-ReLU terminal abstraction under Setting 3 evaluated by solving Eq. … view at source ↗
Figure 5
Figure 5. Figure 5: Local GELU ablation improves concept abstraction and linear probe generalization in transformers. We apply Theorem 8 to transformers by using the local GELU ablation technique described in §5. (A) For visual concepts, we evaluate DINOv3 ViT-L/16 [39] on the 3dshapes task described in §3.3. Local GELU ablation improves or preserves shape abstraction in every layer compared to baseline. Also, baseline abstra… view at source ↗
Figure 6
Figure 6. Figure 6: Abstraction increases along macaque ventral visual stream. We test the prediction of Theorem 7 that deeper cortical layers should exhibit greater abstraction using a subset of recordings in macaque V4 and IT from the public Majaj-Hong dataset [41]. (A) We construct a 2 × 2 factor design around the concepts of naturalness and limbedness. (B) In a pooled site-matched analysis, both concepts are significantly… view at source ↗
Figure 7
Figure 7. Figure 7: Variable-projected readout versus ordinary equal-timescale readout training. We compare full-batch gradient descent on the full loss L(Wr, Z) against gradient descent on the reduced loss L ∗ (Z) = minWr L(Wr, Z), corresponding to Assump. 1. Curves show mean ± SEM over 12 random seeds. A. Shape abstraction α(t). B. Training loss. C. Test accuracy. D. Loss gap for the full-gradient-descent baseline. The vari… view at source ↗

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